In Quebec in 1943, the US and the UK agreed that any use of nuclear weapons would require both countries’ prior approval. The British government gave its formal assent to the US bombing of Hiroshima and Nagasaki at the Combined Policy Committee meeting in Washington on 4 July 1945. Deliberations over the decision were remarkably perfunctory. On 30 April, Field Marshal Henry Maitland Wilson had written from Washington that the US was eager to know British views. The ensuing discussions focused only on the phrasing of London’s assent.

You could argue that British agreement was a formality, as the Americans were going to proceed regardless, but it needn’t have been. After the Quebec Conference, Sir John Anderson – who was in charge of nuclear co-operation with the US – had argued for a thorough consideration of the best way to handle the linked issues of the bomb’s potential wartime use and its long-term international control. In October 1943, Anderson discussed the matter deeply with Niels Bohr, who had just escaped to Britain from the continent. Their conversations resulted in Bohr’s proposal to forestall a nuclear arms race by bringing the Soviets fully into the picture as soon as possible.

Churchill, however, refused to contemplate such a policy. In March and April 1944, he rebuffed Anderson’s entreaties to launch a study into how to handle the diplomatic ramifications of the new weapon. He agreed to meet Bohr and his own scientific adviser, Lord Cherwell, on 16 May. But at the meeting he was dismissive, even rude. ‘It was terrible,’ Bohr later remarked. ‘He treated us like two schoolboys.’ A later meeting between Bohr and President Roosevelt was less unpleasant, but no more productive.

At Hyde Park, New York in September 1944, Churchill and Roosevelt decided that the world should not be informed of the new invention before its first use, which they suggested might be against Japan. They also agreed to indefinite bilateral collaboration on both military and commercial applications of nuclear energy. ‘Enquiries should be made regarding the activities of Professor Bohr,’ they added, ‘and steps taken to ensure that he is responsible for no leakage of information, particularly to the Russians.’ The Cold War was already beginning to take shape.